1. Technical Field
The present invention relates to a voltage controlled oscillator and, more particularly, to a voltage controlled oscillator for an upconverter/downconverter for oscillating at a transmission local oscillating frequency during a transmission mode and at a reception local oscillating frequency during a reception mode, the transmission/reception modes being alternative with each other in a digital radio communication system.
2. Background Art
In early radio communication systems a frequency division multiple accessing (FDMA) scheme of an analog format was used to provide multiple and simultaneous transmission to a single transponder. The FDMA scheme permits carrier waves that are radiated from different earth stations to use the same satellite transponder and allows each carrier to be distinguished from the others when received at an earth station. Recently a code division multiple access (CDMA) scheme and a time division multiple access (TDMA) scheme of a digital format are becoming more popular in today's radio communication systems. For example a recently developed global system for mobile communication known as GSM is a digital communication system having a combination of FDMA and TDMA. Generally, GSM consists of many subsystems, such as a mobile station, a base station subsystem, a network and switching subsystem, and an operation subsystem. GSM has separate transmission and reception frequencies which are different depending upon respective active modes. For example, in the mobile station, the range of a downlink transmission frequency is 890-915 MHz and uplink reception frequency is 935-960 MHz which is generally 45MHz higher than the corresponding downlink transmission frequency. Both the 935-960 MHz uplink and 890-915 MHz downlink have been allocated 25 MHz. Accordingly the bandpass of the downlink transmission frequency is generally 25 MHz, and the frequency divided at intervals of 200 KHz for each voice band channel is used to dedicate to a single destination. Further, GSM operates in an alternative transmission mode and reception mode within each frame, i.e., within a time unit.
Typical GSM requires two separate voltage controlled oscillators (VCO) each used as a local oscillator for an upconverter which oscillates at a transmission frequency during a transmission mode and a local oscillator for a downconverter which oscillates at a reception frequency during a reception mode. Each of the voltage controlled oscillators (VCO) for an upconverter and a downconverter in the GSM typically provides a local oscillation frequency having a frequency range which varies linearly as a function of an input control voltage. Therefore the VCO must be designed for good linearity in generating a local oscillation frequency as a function of an input control voltage. In addition the VCO must be designed to operate at a broad range of frequency.
Conventional voltage controlled oscillators are available in frequencies ranging from audio frequencies to hundreds of megahertz. For example, simple LC-controlled oscillators have a tuned LC connected in an amplifier-like circuit to provide gain at its resonant frequency. Overall feedback is then used to cause a sustained oscillation to build up at the LC's resonant frequency. LC-controlled oscillators have two popular configurations such as the Colpitts oscillator, a parallel tuned LC at the input, with positive feedback from the output as disclosed, for example, in U.S. Pat. No. 4,593,255 for Varactor Tuned Colpitts Oscillator With Compensating Varactor For Wide Bandwidth issued to Matsuura, U.S. Pat. No. 4,621,241 for Wide Range Electronic. Oscillator issued to Kiser, U.S. Pat. No. 5,144,264 for Wideband Voltage Controlled Oscillator Having Open Loop Gain Compensation issued to Chong et al, and U.S. Pat. No. 5,486,790 for Oscillator Circuit For Receiving A Wide Frequency Band Signal issued to Ishikawa et al., and the Hartley oscillators disclosed, for example, in U.S. Pat. No. 4,091,339 for Oscillator issued to Sugimoto et al.
Tunable oscillator modules use varactors (voltage-variable capacitors) to adjust the operating frequency of an LC oscillator in response to an externally applied voltage may be found in U.S. Pat. No. 4,270,098 for Means For Linearizing A Voltage Variable Capacitor Controlled Oscillator issued to Bowman, U.S. Pat. No. 4,450,416 for Voltage Controlled Oscillator issued to Mears, U.S. Pat. No. 4,500,854 for Voltage-Controlled RF Oscillator Employing Wideband Tunable LC Resonator issued to Meyer, U.S. Pat. No. 4,533,881 for Rapid Starting Variable Frequency Oscillator With Minimum Startup Perturbations issued to Monett, U.S. Pat. No. 4,731,592 for Voltage Controllable Oscillator issued to Sato et al.
In the GSM, a conventional VCO is normally required as a local oscillator for each of an upconverter and a downconverter in a radio communication system. Recently, however, an integrated VCO has been designed for both the upconverter and the downconverter so as to reduce the size and cost of the communication system, as disclosed in Korean patent application No. 92-9028 for Voltage Controlled Oscillator, filed on May 27, 1992 by the same assignee, Samsung Electronics Ltd. as the present invention, and Korean patent application No. 94-15043, filed on Jun. 28, 1994 by the same assignee, Samsung Electronics Ltd. as the present invention which is now incorporated by reference herein for purposes of illustrating the state of the art. Generally, this type of VCO is used as a local oscillator for the upconverter during the signal transmission and for the downconverter during the signal reception. This type of VCO, however, has an unsatisfactory frequency range to cover both the transmission frequency bandpass and the reception frequency bandpass. While there are a number of known techniques for extending the range of oscillation frequency of the conventional VCO, I have found that none can minimize the phase noise. Moreover, when the range of oscillation frequency is extended, it is my observation that impedance of the frequency resonator and the negative resistance generator in the conventional LC oscillator must be unduly matched.